Inorganic colorants are widely employed in various applications such as paints, plastics, ceramics, rubbers, enamels and glasses. These pigments may impart coloristic properties and protect the coatings from the effects of visible as well as ultraviolet and infrared light. For such applications, their properties like chemical and thermal stability, dispersibility, chromaticity, tint strength and covering or masking power are particularly important criteria to be taken into account in the selection of a suitable colorant. Unfortunately, the inorganic pigments which are suitable for such applications and which are today actually employed on an industrial scale generally comprise toxic metals (cadmium, lead, chromium and cobalt) (References may be made to “High Performances Pigments,” ed. by H. M. Smith, Wiley-VCH, Weinheim, 2002). The coloring scale of red and yellow inorganic pigments is completely covered by the cadmium sulfoselenides. In the blue and green range besides the ultramarine pigments there are primarily offered cobalt and chromium pigments. The use of above pigments is becoming increasingly strictly controlled, or even prohibited by legislation in many countries, due to their toxicity which is widely known to be very high. Thus serious economic and industrial need continues to exist for substitute inorganic pigments devoid of the above advantages and drawbacks.
Rare earth elements offer a vast opportunity for the development of environmentally secure alternatives for many of the eco-constrained colorants. Rare earths, because of their unique electronic configuration of partially filled f-orbitals, show unusual magnetic and optical properties. The intense coloration of rare earth based materials can arise from mostly charge transfer interactions between a donor and acceptor with the metal ion playing generally the role of an acceptor. Dopants based on rare earth elements in mixed oxide system offer an opportunity to tune the color response through the manipulation of energy gaps and delocalization phenomena in conduction and valence bands. Thus this phenomenon offers scope for design of colorants for specific applications.
However, the separation of rare earths offers a formidable challenge in the field of separation science in view of their similar physico-chemical properties. This in turn results in high costs for obtaining pure individual rare earths. Thus the present invention provides an economic option to the Rare Earth Industry due to the use of mixed rare earth compounds for the manufacturing of green colorant.
Pure rare earth oxides/compounds have been widely employed for the manufacturing of wide range of colorants in the Pigment Industry. U.S. Pat. No. 6,582,814, Jun. 24, 2003 describes a novel process for the synthesis of rare earth-transition metal oxide pigments, having the general formula: (RExTm)Oy, where RE is rare earth, Tm is transitional metal, x ranges from 0.08 to 12 and y ranges from x+1 to 2x+2, for use in plastics, paints, coatings, glass enamels and other materials with various advantages over the traditional pigment formulations. However, in this process green pigments are obtained using toxic metals like chromium.
Thermally/chemically stable and nontoxic inorganic pigments/colorants, characteristically green and well suited for the coloration of a wide variety of materials and substrates, for example, plastics, ceramics, etc. comprising at least one mixed oxide of the formula: Y2BaCuO5, Sm2BaCuO5 and Yb2BaCuO5 has been reported in the U.S. Pat. No. 6,284,033, Sep. 4, 2001.
New ecological green pigments based on Ca—Nd/Y—S system above mentioned application, in continuation with them have been well documented elsewhere for applications into plastics and paints (References may be made to M. D. Hernandez-Alonsoa, A. Gomez-Herrerob, A. R. Landa-Canovas, A. Duran, F. Fernandez-Martinez, L. C. Otero-Diaz, J. Alloys Compounds. 2001, 323-324, 297-302; E. U. Garrote, F. F. Martinez, A. R. L. Canovas, L. C. O. Diaz, J. Alloys Compounds. 2006, 418, 86-89; U.S. Pat. No. 5,501,733, Mar. 26, 1996).
The majority of the processes so far reported in the prior-art for the production of green inorganic pigments utilize pure rare earth compounds. However, recently Sreeram et. al. Reference may be made to WO2006/067799 A1, Jun. 29, 2006, discloses a process for the preparation of green inorganic colorant by employing mixed rare earth compounds (cerium in the range: 40-45% w/w, praseodymium in the range: 4-6% w/w, lanthanum in the range: 15-25% w/w, neodymium in the range: 15-20% w/w and other rare earths to a maximum of 5%) and nickel carbonate. However, the main drawback of this process is not economical and not environmental friendly as it contains nickel as toxic element.
There is no prior information available on the use of mixed rare earth compounds by suitable combination with molybdenum for the synthesis of green pigment.